This invention concerns the safety of moving industrial machines. There is a continuing need to provide safety to workers operating these machines. Because it is impractical to place all moving parts of manufacturing apparatuses behind permanent barriers, various techniques have evolved that tend to force safe operation of these machines.
One safety technique used in industrial machines requires that both hands of each machine operator be positively located a safe distance from the machine's moving parts in order for the machine to cycle. Generally, each operator must simultaneously press two push buttons to initiate operation of the machine. Each pair of push buttons is spaced far enough apart to force each operator to use both hands. Release of any one of these push buttons during the closing operation of the machine stops the drive mechanism. Every push button is positioned so far from the machine that the closing operation is stopped before any of the operators' hands can reach a region in the machine where a hand could be trapped or crushed.
One problem with this technique is determining whether the location of each switch provides safety. This determination is based on the maximum distance an operator's hand could move during the time required to completely stop the machine. Standards for hand speed are used in the computation of the minimum distance between each safety push button and any dangerous region (danger zone) of the machine. Such computations require an accurate determination of the interval between the instant one of the push buttons is released and the instant the machine comes to a complete stop. Reliable measurement of this time period is often difficult.
A special purpose measurement apparatus is known in the industrial machine art. The special purpose measurement apparatus measures the time between the initiation of a sequence to release the safety push button and the stoppage of the machine. An electro-mechanical device is placed over one of the safety push buttons. This special purpose apparatus is known as an artificial hand. The artificial hand is manipulated to depress the one safety push button while the other push buttons are being depressed manually in order to start the machine cycle. Upon receipt of a trigger signal, the artificial hand releases the push button. This trigger signal also starts a timer. Motion of the machine is measured using a tachometer attached to the machine via a spring wound cord. While the machine is moving, the tachometer produces a variable d.c. voltage signal whose magnitude corresponds to the industrial machine speed. Machine motion is adjudged stopped when the d.c. signal decreases below a certain level. The timer is stopped when the machine motion has been adjudged stopped. The elapsed time and a standard hand speed determine the minimum safe distance between the safety push buttons and any danger zone of the industrial machine.
There is a flaw associated with the measurement technique described above. There is an inherent time delay between the generation of the trigger signal, which both starts the timer and initiates the release of the one push button, and the actual movement of the safety push button. This release process involves several steps, each of which introduce some variability into the measurement. The trigger device first energizes a relay. This relay, when closed, supplies power to a solenoid in the artificial hand. The solenoid pulls a release mechanism that releases pressure on the safety push button. The safety push button rises because of an internal spring. Because of these delays, the time measured does not accurately represent the time between the release of the one push button and the stoppage of the machine. This variability in the measure of the time to stop the machine causes undesirable uncertainty. Therefore the art needs an automatic testing apparatus for the machine safety system that reduces or eliminates these sources of variability.